44 CHEMICAL PHYSICS. 



molecules are held rigidly in a fixed position and can vibrate only 

 back and forth. The molecules thus have a certain energy of motion 

 which is called kinetic energy, and in liquids and solids an energy of 

 position also, known as potential energy. This potential energy is 

 due to the position or molecular arrangement of the particles, and to 

 make a change in this necessitates the overcoming of the forces 

 which hold the molecules in place by an expenditure of energy from 

 some outside source. 



Thus it is believed that whenever heat energy is added to a body 

 it either goes to increasing the motion of molecules i. e., the kinetic 

 energy, or to making a change in the relative positions of the mole- 

 cules i. e., in their potential energy, or to both. 



When the motion of the molecules is increased i. e, when the 

 kinetic energy is increased there is a rise in temperature, which we 

 can measure by a thermometer. What we call temperature is the 

 degree or intensity of the sensible heat of a body. 



On the other, hand, heat is absorbed whenever solids pass into the 

 liquid state ; or liquids into the gaseous state. This fact is often 

 made use of in producing artificial cold. Thus, liquefied ammonia is 

 employed in the manufacture of ice, the required low temperature 

 being produced by evaporation of the ammonia. Similarly, the heat 

 absorbed during the liquefaction of snow or powdered ice by an 

 admixture of common salt and the liquefaction of the latter through 

 solution serve for generating a low temperature. The action of the 

 various freezing-mixtures depends on this principle. 



Latent heat. Sometimes heat may be added to a body without 

 any change of temperature, as above in the case of the melting of ice 

 and the boiling of water. In such cases it is believed that the heat 

 added is absorbed in changing the relative arrangement of the mole- 

 cules, which change must evidently take place in passing from solid 

 to liquid water and from liquid to gaseous water. This will account 

 for the apparent loss of heat in melting ice or in boiling water. It 

 is lost only to our senses, and exists in another form as potential 

 energy of the molecules ; hence it is called latent heat. 



This latent heat is given out again when the molecules return to 

 their former arrangement. Hence steam in condensing to water, and 

 water in assuming the solid state, give off large quantities of heat. 



Sources of heat. Our principal source of heat is the sun. Other 

 sources are : The interior of the earth, the high temperature of which 

 is made manifest by the existence of volcanoes and by the increase of 



